JE Pistons is bringing the benefits of asymmetrical skirt design to the popular LS engine platform. Previously offered only for a broad base of high-revving performance import engines, asymmetrical pistons are now available for race and street applications across the GM LS engine architecture. 

If you’re not familiar with asymmetrical pistons, you’ll easily grasp the concept with one glance at the accompanying photos. One side skirt is noticeably larger than the opposite. This design takes advantage of an engine dynamic — whereby piston force against the cylinder wall is greater on the piston’s major thrust side, which is related to the direction of engine rotation. More surface area is required to accommodate that higher thrust loading, but the opposite doesn’t see those forces, and therefore doesn’t require as much surface area.

Skirt view clearly reveals different skirt sizes, forced pin oilers and accumulator groove between the top and second ring to provide pressure relief.

Engine builders will see a lot more bearing life.–Nick DiBlasi, JE Pistons

“The disparity in skirt loading can vary in magnitude, depending on key variables, including the engine’s stroke, rod length and peak cylinder pressure,” says Dane Kalinowski, engineering manager at JE Pistons.

The major thrust surface is the side of the cylinder that the piston is forced against during the power stroke. It is always 90 degrees to the piston-pin axis. The major thrust in LS engines is against the exhaust side of the cylinders on the right/passenger bank and the intake side of the cylinders on the left/driver bank. When the piston changes direction at TDC to begin the power stroke, combustion typically peaks about 10-25 degrees after TDC. Peak pressure rolls the piston toward the major thrust side where the skirt absorbs the force as the piston move downward, distributing the load and maintaining piston stability.

Understanding thrust force

Engineers have recognized this thrust force since the earliest days of the internal combustion engine; hence, piston skirts are specifically designed to ensure stability for the piston and, more importantly, the ring pack in the cylinder bore. Without skirts the pistons would rock in the bore, compromising ring seal and piston durability. Full-round skirts were the primary design for many years. They are still employed today in most automotive and industrial engines, but the JE pistons use a forged-side-relief design (FSR) that incorporates major and minor thrust skirts, inboard pin bosses and a dramatically shortened and stiffer wrist pin. Over the years, the skirt designs were tweaked, sometimes significantly, as designers refined their function according to changing performance requirements.

JE’s new asymmetrical pistons for LS motors bring the latest performance innovations to GM’s popular LS engine series in both race and street versions.

Asymmetrical design theory reduces the size of the skirt on the minor thrust side of the piston where less loading is experienced. Reductions in friction and piston weight are the primary benefits, but there’s more to it than first meets the eye. Understanding the subtle engineering intricacies of piston design require some basic discussion of piston shape, contact pressure, thermal and mechanical stress, ring position and so on. It may help to visualize a piston and ring package much like a catamaran boat with outriggers. The rings and ring land area represent the main hull and the skirts are the outriggers that provide stability.

“The real trick is the asymmetrical structure itself,” affirms Nick DiBlasi, product manager at JE Pistons. “We are specifically designing the structure of the piston to handle the stresses on the part as it’s experienced in the engine. And a big benefit is weight. Engine builders will see a lot more bearing life.”

Ring stability and seal is the absolute goal. To minimize friction, designers strive to limit the amount of piston material that actually contacts the cylinder walls. Most pistons are not perfectly round with some exception in the ring land area.  Below the oil ring, pistons are oval shaped  to evenly distribute contact pressures between the piston and cylinder wall. This oval shape is called cam grinding and it shapes the piston to provide the desired amount of contact patch against the cylinder wall with contact centered on the crown of each thrust surface.  On the vertical axis the skirt is shaped like the side of a barrel with a reduced diameter at the top just below the oil ring. The skirt  deflects ever so slightly under high loading to provide stability via the increased contact surface.

Measuring side loads

When a manufacturer designates a specific location for measuring piston to wall clearance, it is targeting the specific area of skirt contact. Radiating out from that point, parts of the skirt may not contact the cylinder wall. The polished area you see on most used piston skirts reveals the actual contact patch. The main focus of the asymmetrical piston design is weight and friction reduction via the FSR’s smaller, lighter skirt on the minor thrust side. To ensure optimum skirt profiles, JE assumed very high cylinder pressures based on typically demanding forced induction applications of more than 1,000 horsepower. For the LS engine, they modeled a 4.100-inch stroke, 6.125-inch rod peak-firing-pressure scenario. 

The side load chart indicates load difference between major and minor thrust surfaces. Forces are indicated in Newtons. Immediately after piston reversal major thrust loading approaches 12,000 N while the minor thrust load is barely 3000 N.

The accompanying LSX Side Load Chart illustrates the difference between major and minor thrust loads on the piston skirts for the same application. As depicted, the major thrust load force is close to 12,000 N (Newtons) while the minor thrust load is barely 3,000 N. The difference clearly invites an engineering solution tailored to accommodate different loading on each side of the piston, hence the asymmetrical skirt design.

In the accompanying FEA (finite element analysis) images the stress on both skirts appears similar because each skirt has been designed for its own particular stress. The analysis clearly shows safe stress levels on both the major and minor skirts despite the disproportionate loading they actually experience. As a point of reference, 1 N is approximately 0.22481-pound-force, which means that the side loading on the major thrust side is nearly 2,700 pounds at peak pressure.

“Initial designs were accomplished in SolidWorks modeling software, then subjected to multiple iterations of  finite element analysis prior to validation testing in high-horsepower, turbocharged engines with successful results at power levels up to 2,000-plus horsepower,” Kalinowski explains. 

Left photo shows the piston stress analysis on the major thrust side. Note highest concentration of load against the center of the skirk. Loading on the minor thrust side, right photo, appears similar because each skirt is specifically designed to accommodate the stress it is experiencing; hence the analysis shows safe stress levels on both skirts, despite the dramatic difference in loading.

Asymmetrical piston benefits

A “clean sheet” approach presented a parallel opportunity to further refine piston features. Additional built-in attributes include inboard pin bosses with shorter, lighter race-style wrist pins and offset pin location to achieve proper piston balance. There’s also a contoured crown thickness to maximize piston stiffness and durability along with a smooth finish to the crown surface. 

FEA (finite element analysis) stress on the piston under peak firing pressure indicates maximum loading at the center of the piston crown, around the pin bosses and against the center of the piston skirt. View of the undercrown shows high stress accumulated around the pin bores, an indication of the need for substantial pin strength particularly in forced induction applications.

Shorter Piston Pins

One of the key design benefits of the asymmetrical piston is the shorter and lighter piston pin. The photo above illustrates the difference between a standard pin and the one used in asymmetrical pistons. In fact, the LS piston pins measure 2.250 inches and reduces piston weight by up to 10 grams. They are also secured with round wire locks (shown below).

Pin offset has long been used in production engines to combat piston slap and reduce noise. In this case the pins are offset approximately .020-inch toward the larger skirt to maintain static and dynamic balance. Most asymmetrical pistons use round wire locks for the wrist pins. These are all high performance pistons; hence, there are no pressed pin versions.
 
The contoured crown thickness is configured to accommodate varying thermal and mechanical (pressure) stress levels concentrated near the center of the crown and tapering off toward the edges. The thicker reverse crown on the underside of the piston deck gradually tapers toward the ring lands according to observed stress levels. This provides strength in the center where it is necessary while minimizing material toward the outside to help reduce piston weight. 

“It has a really robust crown,” confirms DiBlasi. “The pistons are made from 2618 high-strength, low-silicone aluminum alloy — which is the same for all race pistons, including our billet line.”

Nominal compression ratio for domed pistons is 13.2:1 with flat top versions ranging from 10.6:1 to mid 12’s and dished models yielding 9:1 CR with max engine size of 439 cubic inches. Virtually every size, compression ratio and rod length is now achievable across the LS engine family with asymmetrical pistons.

JE’s asymmetrical FSR flat tops also feature accumulator grooves and double-forced pin oilers in the oil ring groove. Within the LS engine family, JE now offers flat top and inverted dome asymmetrical pistons for LS1, LS6, Z06, 5.7L and 6.0L engines plus a 15° LS1 version and flat-top, dished and domed versions for LS and LS7 engines. Bore sizes include 4.005, 4.030 and 4.125-inch for a 4.00-inch stroke and a 6.125-inch rods. The 15° LS1 pistons are spec’d for a 3.905-inch bore with a 3.622, 3.900 or 4.000 stroke with appropriate rod length. Domed versions allow up to 440 cubic inches with a 4.185-inch bore and 6.125-inch rods.

Crown side view shows the curvature of the crown with thickness concentrated toward the center for maximum strength at the highest load point.

“All the dish and flat-top pistons will work with any head that physically bolts to that block,” says DiBlasi, noting that pricing has actually come down for the asymmetrical line and that there will be up to 160 total part numbers for the shelf. “Dome pistons only fit 15-degree heads. If someone has a 12-degree and needs a dome, we have to custom make it. Otherwise, all 12- and 15-degree flat-top or dish can be accommodated off the shelf.”

Blown Z's JE Asymmetrical Pistons

JE’s new asymmetrical pistons were used in the winter rebuild of project Blown Z’s 388ci LSX engine. Note the comparison of the minor thrust side of the new vs. old piston above (click on the image for a fullsize view). The full story can be found here.

 The lightest asymmetrical piston is the 382 gram 383-stroker piston. The heaviest is the 427ci version at 440 grams. Most off-the-shelf weights are fixed in common bore sizes, but the bulk of the available pistons require you to call for the spec — as it depends on the bore size, valve reliefs, dome cc’s and so on. The ring package and pin diameter also vary depending on the application. Most applications use a 1.2mm, 1.5mm, 3.0mm ring pack or a 1.5mm, 1.5mm and 3.0mm ring pack. All pistons assume a .040-.042-inch head gasket with zero deck clearance. Skilled builders can achieve higher compression ratios by juggling deck height, valve pockets and further reducing chamber volume although it’s not recommended unless you’re well versed in mating these components properly.

Among the major concerns of engine builders adequate clearance for the reluctor wheel on the crankshaft ranks high. All JE asymmetrical pistons provide proper reluctor wheel clearance, but the oil jets on the LSA and LS9 blocks require slight modification for proper clearance. Gas porting is an available option for extra cost.

JE first developed the asymmetrical piston for NASCAR and a leading road-racing team before “bringing the concept downstream,” first to the import market and now to the LS consumer to accommodate the widespread popularity and adoption of these modern engines. The company is also rolling out a non-sysmmetrical version for the LS1. Conventional small- and big-blocks families are likely to follow. And at some point Ford and Mopar pistons will also receive the asymmetrical treatment.